1. Field of the Invention
The present invention relates to a frequency modulation radar apparatus for vehicle use which is installed on a motor vehicle for measuring the distance, the relative speed, etc., to a neighboring vehicle for the purpose of being used for vehicle-to-vehicle distance control, collision damage reduction, etc.
2. Description of the Related Art
Conventionally, there has been well known a frequency modulation radar apparatus for vehicle use which sends a transmission signal the frequency of which is frequency modulated so as to linearly rise or fall in accordance with the elapse of time, receives the signal reflected by a target object, samples a beat signal that is obtained by mixing the reception signal thus received with the transmission signal, performs a discrete frequency analysis by using a predetermined window function, extracts a peak signal from the spectrum of a beat signal thus obtained, and calculates a distance or a relative speed to the target object based on the frequency of the peak signal.
As such a kind of frequency modulation radar apparatus for vehicle use, there has generally been known one that is constructed as shown in a block diagram of FIG. 8 for example. In FIG. 8, the frequency modulation radar apparatus for vehicle use is composed of a CPU 1, a modulation voltage generation part 2, a voltage controlled oscillator (VCO) 3, a circulator 4, an antenna 5, a mixer 6, an A/D converter 7 and an FFT (fast Fourier transform) calculation section 8.
When an instruction for starting modulation is given from the CPU 1, the modulation voltage generation part 2 impresses a triangular wave voltage to the VCO 3. The VCO 3 frequency modulates a carrier wave according to the triangular wave voltage, and transmits it to the antenna 5 as an transmission signal through the circulator 4, and at the same time inputs the transmission signal to the mixer 6. The antenna 5 radiates the transmission signal as a radio wave toward a target object X, receives the radio wave reflected by the target object X and transmits it to the circulator 4 as a reception signal. The circulator 4 inputs the reception signal to the mixer 6 which mixes the reception signal and the transmission signal with each other thereby to output a beat signal. The A/D converter 7 samples the beat signal and inputs the result of the sampling to the FFT calculation section 8. The FFT calculation section 8, after multiplying the beat signal thus sampled by a predetermined window function, performs fast Fourier transform (FFT) on the sampled beat signal thus multiplied.
FIG. 9 is an explanatory view that shows one example of the spectrum of the beat signal, wherein the axis of abscissa represents the frequency [x 1/T] of the beat signal, and the axis of ordinate represents the strength of the beat signal, with the frequency of a true peak signal being designated by “ft”.
In FIG. 9, T in the unit [1/T] on the axis of abscissa is an observation time, individual points indicated by round marks represent a discrete spectrum of the beat signal, and solid lines represent envelopes of the discrete spectrum (i.e., continuous spectrum shapes corresponding to the window function).
When the spectrum of the beat signal is in a state shown in FIG. 9, the FFT calculation section 8 transmits a frequency fn and a strength an (peak value) of the peak signal in the discrete spectrum to the CPU 1 as the frequency and strength of the beat signal. Accordingly, the CPU 1 calculates the distance, the relative speed, etc., of the target object X based on the frequency fn and the strength an of the peak signal, and outputs them to an external device (not shown) as target object information Y.
At this time, the frequency interval of the discrete spectrum according to the fast Fourier transform is represented by a frequency resolution 1/T (the reciprocal of the observation time T), so the frequency fn of the peak signal in the discrete spectrum generally has an amount of deviation (corresponding to an amount of frequency correction to be described later) δ [1/T] with respect to a frequency ft of the true peak signal. Here, if the frequency resolution 1/T is to be set to a small value so as to detect the target object X with a high degree of precision, the observation time T will naturally increase and the processing time will also increase, and hence this is not desirable.
Accordingly, in the past, there has been proposed a radar apparatus that serves to cope with the above-mentioned problem (see, for example, a first patent document: Japanese patent application laid-open No. 2003-240842). In this first patent document, the frequency spectrum of a window function is fitted to the discrete spectrum of a beat signal whereby a target object X is detected by making a peak of the window function thus fitted as a true peak. Alternatively, an amount of deviation δ between the frequency fn of a peak signal of the discrete spectrum and the frequency ft of a true peak signal is beforehand represented as a function of the strength of the discrete spectrum based on a window function, and the amount of deviation δ is calculated from the strength of the discrete spectrum obtained by fast Fourier transform.
In the radar apparatus described in the above-mentioned first patent document, specifically, in case of using a Hanning window, it is assumed that a strength ratio ΔP between strengths at the opposite adjacent sides of the frequency ft of the peak signal in the discrete spectrum is represented by the following expression using the strengths (an−1, an+1) of the discrete spectrum at the opposite adjacent sides of the frequency ft of the true peak signal in FIG. 9.ΔP=an+1/an−1 
At this time, the logarithm ΔPdB of the strength ratio ΔP is represented by the following expression (9).
                              Δ          ⁢                                          ⁢                      P            dB                          =                              20            ⁢                                                  ⁢                          log              ⁡                              (                                  Δ                  ⁢                                                                          ⁢                  P                                )                                              =                      20            ⁢                                                  ⁢            log            ⁢                                                            (                                      1                    +                    δ                                    )                                ⁢                                  (                                      2                    +                    δ                                    )                                                                              (                                      1                    -                    δ                                    )                                ⁢                                  (                                      2                    -                    δ                                    )                                                                                        (        9        )            
Accordingly, the amount of deviation δ is calculated by the following expression (10) which linearly approximates the relation of the above expression (9).δ=3.67×10−2ΔPdB  (10)
In the conventional frequency modulation radar apparatus for vehicle use, in actuality, there exists noise resulting mainly from thermal noise, so the signal strengths an−1, an, an+1 of the discrete spectrum obtained by fast Fourier transform become different from the values theoretically predicted, and hence there arises a problem that an error occurs in the estimated value of the amount of deviation δ that is calculated in the manner as shown in the above-mentioned first patent document. In addition, there is also another problem that a correction method, though required for reducing the influence of noise so as to ensure sufficient accuracy, can not be achieved. Moreover, there is a further problem that a linear approximation, being used in the correction of the frequency fn of the peak signal, becomes a cause for the occurrence of an undesirable error.